FAILED: patch "[PATCH] serial: amba-pl011: keep transmitter enabled when disabling RS485" failed to apply to 5.15-stable tree

FAILED: patch "[PATCH] serial: amba-pl011: keep transmitter enabled when disabling RS485" failed to apply to 5.15-stable tree

[Sasha Levin]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 97f85204275ae4d7fb530e451f3acc65c1ec1155
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107060610-failed-5.15-97f85204275a@kernel.org' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 97f85204275ae4d7fb530e451f3acc65c1ec1155 Mon Sep 17 00:00:00 2001
From: Nicolai Buchwitz <nb@tipi-net.de>
Date: Mon, 12 Oct 2026 16:23:50 +0200
Subject: [PATCH] serial: amba-pl011: keep transmitter enabled when disabling
 RS485

Disabling RS485 on an open port calls pl011_rs485_tx_stop(), which
turns off the transmitter. Only the RS485 TX path turns it back on, so
any later output stays queued and tcdrain() hangs until the port is
closed.

pl011_rs485_tx_stop() also checks only the hardware FIFO. If a DMA
transfer has started but has not filled the FIFO yet, the transmitter
is turned off under the transfer and the DMA channel never recovers.

When leaving RS485 mode, cancel pending deferred starts and stops, leave
the transmitter enabled and start any TX that was still waiting for
delay_rts_before_send.

Fixes: 8d479237727c ("serial: amba-pl011: add RS485 support")
Cc: stable@kernel.org
Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20261012-fix-pl011-rs485-txe-v2-1-e9bf7d218389@tipi-net.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/serial/amba-pl011.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index cd8cbc9672815..3edd83ec73de1 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -2398,8 +2398,31 @@ static int pl011_rs485_config(struct uart_port *port, struct ktermios *termios,
 	struct uart_amba_port *uap =
 		container_of(port, struct uart_amba_port, port);
 
-	if (port->rs485.flags & SER_RS485_ENABLED)
+	if ((port->rs485.flags & SER_RS485_ENABLED) &&
+	    !(rs485->flags & SER_RS485_ENABLED)) {
+		bool tx_pending = uap->rs485_tx_state == WAIT_AFTER_RTS;
+		u32 cr = pl011_read(uap, REG_CR);
+
+		/*
+		 * Leaving RS485 mode, which only enables the transmitter while
+		 * sending. Don't stop it, as that stalls a DMA transfer still
+		 * in flight. Drop pending deferred starts and stops and keep
+		 * it enabled.
+		 */
+		hrtimer_try_to_cancel(&uap->trigger_start_tx);
+		hrtimer_try_to_cancel(&uap->trigger_stop_tx);
+		uap->rs485_tx_state = OFF;
+
+		cr |= UART011_CR_TXE | UART011_CR_RXE;
+		pl011_write(cr, uap, REG_CR);
+
+		/* Start TX that was still waiting for delay_rts_before_send */
+		if (tx_pending && !uart_tx_stopped(port) &&
+		    !pl011_dma_tx_start(uap))
+			pl011_start_tx_pio(uap);
+	} else if (port->rs485.flags & SER_RS485_ENABLED) {
 		pl011_rs485_tx_stop(uap);
+	}
 
 	/* Make sure auto RTS is disabled */
 	if (rs485->flags & SER_RS485_ENABLED) {
-- 
2.53.0

[PATCH 5.15.y 1/2] serial: amba-pl011: remove redundant code in rs485_config

[Sasha Levin]

From: Lino Sanfilippo <LinoSanfilippo@gmx.de>

[ Upstream commit a9efa452486ed59b38f15c2287d3e7e129f6e7fc ]

In uart_set_rs485_config() the serial core already

- ensures that only one of both options RTS on send or RTS after send is
  set

- nullifies the padding field of the passed serial_rs485 struct

- clamps the RTS delays

- assigns the passed serial_rs485 struct to the uart port

So remove these tasks from the code of the drivers rs485_config() function
to avoid redundancy.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Link: https://lore.kernel.org/r/20220410104642.32195-3-LinoSanfilippo@gmx.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Stable-dep-of: 97f85204275a ("serial: amba-pl011: keep transmitter enabled when disabling RS485")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/tty/serial/amba-pl011.c | 16 +---------------
 1 file changed, 1 insertion(+), 15 deletions(-)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index a5340c5e11b10..7b21907cd14dd 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -2241,25 +2241,11 @@ static int pl011_rs485_config(struct uart_port *port,
 	struct uart_amba_port *uap =
 		container_of(port, struct uart_amba_port, port);
 
-	/* pick sane settings if the user hasn't */
-	if (!(rs485->flags & SER_RS485_RTS_ON_SEND) ==
-	    !(rs485->flags & SER_RS485_RTS_AFTER_SEND)) {
-		rs485->flags |= SER_RS485_RTS_ON_SEND;
-		rs485->flags &= ~SER_RS485_RTS_AFTER_SEND;
-	}
-	/* clamp the delays to [0, 100ms] */
-	rs485->delay_rts_before_send = min(rs485->delay_rts_before_send, 100U);
-	rs485->delay_rts_after_send = min(rs485->delay_rts_after_send, 100U);
-	memset(rs485->padding, 0, sizeof(rs485->padding));
-
 	if (port->rs485.flags & SER_RS485_ENABLED)
 		pl011_rs485_tx_stop(uap);
 
-	/* Set new configuration */
-	port->rs485 = *rs485;
-
 	/* Make sure auto RTS is disabled */
-	if (port->rs485.flags & SER_RS485_ENABLED) {
+	if (rs485->flags & SER_RS485_ENABLED) {
 		u32 cr = pl011_read(uap, REG_CR);
 
 		cr &= ~UART011_CR_RTSEN;
-- 
2.53.0

[PATCH 5.15.y 2/2] serial: amba-pl011: keep transmitter enabled when disabling RS485

[Sasha Levin]

From: Nicolai Buchwitz <nb@tipi-net.de>

[ Upstream commit 97f85204275ae4d7fb530e451f3acc65c1ec1155 ]

Disabling RS485 on an open port calls pl011_rs485_tx_stop(), which
turns off the transmitter. Only the RS485 TX path turns it back on, so
any later output stays queued and tcdrain() hangs until the port is
closed.

pl011_rs485_tx_stop() also checks only the hardware FIFO. If a DMA
transfer has started but has not filled the FIFO yet, the transmitter
is turned off under the transfer and the DMA channel never recovers.

When leaving RS485 mode, cancel pending deferred starts and stops, leave
the transmitter enabled and start any TX that was still waiting for
delay_rts_before_send.

Fixes: 8d479237727c ("serial: amba-pl011: add RS485 support")
Cc: stable@kernel.org
Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20261012-fix-pl011-rs485-txe-v2-1-e9bf7d218389@tipi-net.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[ sashal: Adapted RS485 handling to the existing rs485_tx_started boolean and synchronous delays, omitting unavailable timer and deferred-start logic. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/tty/serial/amba-pl011.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 7b21907cd14dd..3f74b96e7745c 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -2241,8 +2241,22 @@ static int pl011_rs485_config(struct uart_port *port,
 	struct uart_amba_port *uap =
 		container_of(port, struct uart_amba_port, port);
 
-	if (port->rs485.flags & SER_RS485_ENABLED)
+	if ((port->rs485.flags & SER_RS485_ENABLED) &&
+	    !(rs485->flags & SER_RS485_ENABLED)) {
+		u32 cr = pl011_read(uap, REG_CR);
+
+		/*
+		 * Leaving RS485 mode, which only enables the transmitter while
+		 * sending. Don't stop it, as that stalls a DMA transfer still
+		 * in flight. Keep it enabled.
+		 */
+		uap->rs485_tx_started = false;
+
+		cr |= UART011_CR_TXE | UART011_CR_RXE;
+		pl011_write(cr, uap, REG_CR);
+	} else if (port->rs485.flags & SER_RS485_ENABLED) {
 		pl011_rs485_tx_stop(uap);
+	}
 
 	/* Make sure auto RTS is disabled */
 	if (rs485->flags & SER_RS485_ENABLED) {
-- 
2.53.0